Nuclear valve refers to the valves used in the Nuclear Island (NI), Conventional Island (CI) and auxiliary facilities, the balance of Nuclear Island (BOP) system of the power plant. These valves can be divided into class Ⅰ, Ⅱ, Ⅲ, non-nuclear according to its security requirements in sequence. Valves are the most used control equipment transporting flow media and the essential part of the nuclear power plant.
Nuclear Island is the core of a nuclear power plant where nuclear energy is converted into thermal energy, including the Nuclear Steam Supply System (NSSS) and Nuclear Island auxiliary facility (BNI). The NCI is the workhorses of nuclear power plants, where heat is converted into electricity (including steam turbines all the way to power output). The usage of valves in the three systems of NI, CI and BOP is 43.5%, 45%, and 11.5% respectively.
A pressurized water reactor nuclear power plant will need about 1.13 million NI valves, which can be divided into gate valves, globe valves, check valves, ball valves, butterfly valves, diaphragm valves, pressure relief valves and regulating (control) valves according to the types of valves. This section mainly introduces gate valves in nuclear safety (specification) classes Ⅰ and Ⅱ.
The diameter of gate valves for Nuclear Island is generally DN 80mm-350mm. Forgings are suggested; be used for gate valve bodies of class grade Ⅰ and castings are allowed for gate valve bodies of Nuclear class 2 and 3. However, forgings are often used because the casting quality is not easy to control and be guaranteed. The valve body and bonnet of the nuclear valve is usually flange connected, which adds a lip sealing welding process and makes the sealing more reliable. In order to prevent the leakage of the medium, the double layer packing belt is usually adopted, and the Disc spring pretension device is used to prevent the packing loosening. These gate valves can be driven manually or electrically. The influence of the rotational inertia of the motor on the closing force should be taken for electric transmission device of the electric gate valve. It is better to use the motor with a braking function to prevent overload.
According to its body structure, nuclear gate valve can be divided into wedge elastic single gate valve, wedge double gate valve, parallel double gate valve with spring pretension and parallel double gate valve with top block.
Wedge type elastic single gate valve is characterized by its reliable sealing seats and the angle matching between the sealing surface of gate and valve body is required, which is widely used in the main loop system of nuclear power plants. Wedge-type double-plate gate valve is a common valve in thermal power station, its wedge double-plate angle can be adjusted by itself, more reliable sealing and convenient maintenance.
A load of parallel double gate valve with spring preloading will not increase sharply when the gate is closed, but the gate never releases the valve seat made by the spring when it is opened and closed, which leading more sealing surface weariness. The top block parallel type double gate valve provides a more reliable sealing performance which using the top block to make the inclined plane of the two gates stagger to close the gate valve.
Gate valve without packing is also used in the nuclear island. The hydraulically operated gate valve that depends on its own pressurized water to push the piston to open or close the valve. The fully enclosed electric gate valve uses a special motor to operate the gate by means of an inner planet decelerating mechanism which is immersed in water. However, these two gate valves have the disadvantages of complex structure and high cost.
Generally speaking, the features of gate valves for nuclear islands should be:
1) Welded hydraulic double gate plate parallel gate valve with a nominal pressure PN17.5 Mpa, working temperature up to 315℃ and nominal diameter DN350~400mm.
2）Electric wedge-type double gate valve applied in light water coolant primary circuit would be nominal pressure PN45.0Mpa, temperature 500℃ and nominal diameter DN500mm.
3）Electric wedge-type double gate valve used in the primary road of nuclear power plant with graphite-moderated reactor should be nominal pressure PN10.0Mpa, nominal diameter DN800mm and operating temperature up to 290℃.
4) Welded connected electric elastic plate gate valve is adopted on steam and process water pipes of steam turbine plant with a nominal pressure pn2.5mpa, working temperature 200℃, nominal diameter DN100~800mm.
5）The double gate valve with diversion hole is used in the high power graphite-moderated boiling water reactor nuclear power plant. Its nominal pressure is PN8.0MPa while the opening or closing of the valve carried when the pressure drop is ≤1.0MPa.
6) Elastic plate gate valve with frozen sealing packing is ideal for fast reactor nuclear power plant.
7) Internal pressure self-sealing bonnet wedge-type double gate valve for water-hydro power reactor unit with nominal pressure pn16.0mpa and nominal diameter DN500mm.
8) Wedge-type double gate valves with butterfly springs on travel parts are normally bolted flanged and sealed welded.